Problem in declaring the array of structures in managed C#?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
            [StructLayout(LayoutKind.Sequential)]
        public class Details
        {
            public uint ID;
            public uint state;
            public uint country;
            public uint place;
        }
        [StructLayout(LayoutKind.Sequential)]
        public class UserDetails
        {
            [MarshalAs(UnmanagedType.ByValArray, SizeConst=8)]
            public Details[] userDetails;
        }
        static void Main(string[] args)
        {

            UserDetails u = new UserDetails();
            int sizeofDetails = Marshal.SizeOf(u);
        }
    }
}